1. Technical Field of the Invention
This invention relates generally to the design and manufacture of high performance, composite, tubular structures. More specifically, the invention relates to high performance, composite, tubular structures that utilize an integral pattern of reinforcing ribs on the inner diameter (xe2x80x9cIDxe2x80x9d) or outer diameter (xe2x80x9cODxe2x80x9d) surface of the tube.
2. Background of the Prior Art
Thin-walled, high performance, tubular structures have a wide variety of practical uses, such as for graphite composite golf shafts, arrows, bats, ski poles, hockey sticks, bicycle parts and many other applications. Current state of the art, high performance, tubular structures are constructed by various methods and from various materials. Designers of such tubular structures satisfy certain design criteria (such as strength, stiffness, weight and torsional behavior) by varying material types (fibers/resins), orientations of fiber directions and geometric proportions of the tube itself Another way designers have sought to improve high performance tubes is by developing new manufacturing techniques.
Using one manufacturing method, tubular structures are made by rolling material, such as pre-impregnated sheets of fiber/resin (xe2x80x9cprepregxe2x80x9d), onto a xe2x80x9cmandrel.xe2x80x9d The rolled layers of prepreg are then consolidated against the outer surface of the mandrel (called the xe2x80x9cID control surfacexe2x80x9d) by wrapping the prepreg layers with shrink tape and curing via elevated temperature. FIG. 1 is a simplified diagram of this method, which involves wrapping layers of prepreg 108 around a mandrel 102 and wrapping a layer of shrink wrap material 104 around the layers of prepreg 108. Through the application of heat, the shrink wrap material 104 contracts, providing external compaction pressure 106 such that the layers of prepreg 108 are consolidated and cured to form a tubular structure.
FIGS. 2a and 2b are copies of magnified, cross sectional photographs of xe2x80x9cflag wrappedxe2x80x9d (FIG. 2a) and xe2x80x9cfilament woundxe2x80x9d (FIG. 2b) high performance tubular structures made by the method described in FIG. 1. FIGS. 2a and 2b readily demonstrate wall irregularities 202, 204 in tubular structures which frequently result from conventional manufacturing techniques.
The standard flag wrapping and filament wound processes for manufacturing high performance tubes have several drawbacks due to the fact that, during consolidation/curing, the diameter of the tube is reduced. This reduction in diameter typically makes the final OD surface rough and irregular, thus requiring secondary finishing by centerless grinding and sanding. Grinding and sanding make the OD surface of the tubular structure uniform and smooth so that it can be painted to yield a cosmetically acceptable finish. However, the grinding/sanding process also typically cuts and abrades the outermost fibers of a tubular structure. Because these outermost fibers are the most highly stressed due to their location (i.e., "sgr"max=MC/I, where xe2x80x9cCxe2x80x9d is the distance to the outside layer), the grinding/sanding process usually reduces the structural integrity of a tubular structure.
One variation on the flag wrapping and filament wound techniques for making high performance tubular structures is to consolidate them from the inside, rather than the outside, thus yielding a xe2x80x9cnet moldedxe2x80x9d outer surface. This technique uses a female mold, rather than a grinding/sanding process, and the resulting outermost fibers are less distorted during consolidation/cure and are also not cut or abraded during grinding/sanding. The net molding technique also allows for the use of higher, more uniform, consolidation pressures than the conventional, shrink-wrap, flag wrapping and filament wound techniques. Higher consolidation pressures result in higher integrity laminates with fewer voids and, therefore, greater tubular strength.
Though the net molding technique may be an improvement over the shrink wrapping techniques, prior art methods for producing high performance, composite, tubular structures are still limited. One problem, aside from the wall irregularities discussed above, is the inability of prior art tubular structures to attain optimal wall thinness while retaining sufficient tubular strength. Whichever technique is chosen for manufacture, a designer typically strives to produce a tubular structure with a uniform, consistent, well-consolidated wall thickness, with undamaged, undistorted composite fibers. A designer also typically tries to make the wall of the tubular structure as thin as possible, to decrease the weight of the tube, while attaining sufficient wall stiffness and strength to enable the structure to be used for its intended purpose. For example, as the wall of a tubular structure is made thinner, its overall stiffness and strength usually decrease. A fundamental failure mode, such as buckling, of a tubular structure may result if the wall of the structure is too thin. A tube that buckles (typically from compression) cannot achieve its maximum strength. Buckling, in turn, usually leads to further structural failures, such as local fiber breaking and premature catastrophic structure failure.
Structural failure is especially likely if a tubular structure is bent when used for its intended purpose. FIGS. 3a and 3b, for example, show a tubular structure 302 with arrows representing tension 304 and compression 306 forces which might occur with bending 308. The combination of tension 304 on one side and compression 306 on the other side of a tubular structure 302 may cause deflection 310 of the structure, as shown in FIG. 3b. The stiffness of the wall of a tubular structure 302, determined by such factors as the material used to make the tube and the thickness of the wall of the tube, determines how much deflection 310 occurs when the tubular structure is loaded with bending forces. If deflection 310 reaches a certain point, a situation of exponential decay is reached, wherein the stresses present at the wall section increase exponentially until the wall eventually buckles catastrophically. Because instability is inherent in ultra-thin walls of tubular structures, designers generally must use thicker walls than are desirable, in order to achieve adequate stiffness (which translates to adequate stability). Therefore, using prior art methods to produce high performance, composite, tubular structures, the goal of optimal lightness is sacrificed somewhat to achieve requisite stiffness and strength. Accordingly, a long-felt need exists for a high performance, composite tubular structure, and a method for producing that structure, which will combine optimal wall thinness with optimal resistance to buckling and buckling-related stress.
The present invention satisfies the needs described above by providing high performance, composite, tubular structures that are lighter and/or more resistant to buckling-related stress than conventional tubes. In general, the present invention incorporates features into the design of tubular structures to enhance performance.
For example, in accordance with one preferred embodiment of the present invention, tubular structures are enhanced by incorporating small, stabilizing, raised ribs on the ID or OD of the tubes. These ribs enable designers to optimize the tubes"" inertial properties (area mass moments of inertia) to achieve lighter weight, greater stiffness, increased strength or some combination of all three. The ribs may be configured in a variety of shapes and sizes, but are typically helical or circular, parallel or non-parallel, and/or may travel in opposite directions and cross over one another. In accordance with various aspects of the present invention, the ribs may also be hollow. Hollow ribs optionally allow specific materials that are different from the rest of the tubular structure to be included within the ribs. Thus, it will be readily apparent to one skilled in the art that countless combinations and variations of ribs according to one embodiment of the present invention are possible. Like I-beams used in construction, the integral ribs allow designers of tubular structures to use lesser amounts of material, thus optimizing wall thinness, white simultaneously maintaining wall stability and, therefore, strength.